1. Field of the Invention
The present invention relates to a braking device of a measuring instrument and the measuring instrument. More particularly, the present invention relates to a braking device of a spindle that is axially and reciprocably displaceable relative to a case body, and a measuring instrument including the braking device.
2. Description of Related Art
There has been known a dial gauge (mechanical type, digital type) as a measuring instrument that measures a dimension or the like of an object based on a displacement of a spindle that is axially and reciprocably displaceable relative to a case body by bringing a distal end (probe) of the spindle into contact with the object.
In the dial gauge, the distal end of the spindle is biased to project from the case body by a biasing unit. Accordingly, the spindle may vigorously collide with the object by suddenly moving down due to the biasing force. Thus, a braking device has been suggested in which a spindle is prevented from colliding with an object to be measured by braking a sudden movement of the spindle (for instance, see Document 1: JP-A-59-170701).
FIG. 10 shows a traditional braking device of a spindle. A braking device 100 includes: a bush 101 for guiding a second end of a spindle 3 that penetrates a case body 2 and has a probe (not shown) on a first end; a cylinder 102 of which a first end is externally fitted to the bush 101 for accommodating the second end of the spindle 3; a piston 103 provided on the second end of the spindle 3 to define a first chamber R1 between the piston 103 and the bush 101; and a partition 104 fitted to a second end of the cylinder 102 to define a second chamber R2 between the partition 104 and the piston 103.
The spindle 3 is biased downwardly (toward the lower side in FIG. 10) by a biasing unit (not shown).
The piston 103 has an outer diameter that is slightly smaller than an inner diameter of the cylinder 102, so that a clearance having an appropriate size sufficient for a fluid to flow is provided between an outer circumference of the piston 103 and an inner circumference of the cylinder 102.
A cap 105 is fitted to the partition 104 to define a valve chamber R3 therebetween, while a through hole 106 is provided on the partition 104 to communicate between the second chamber R2 and the valve chamber R3. A conical valve seat 107 is provided on the through hole 106 to accommodate a ball 108.
When the spindle 3 is moved down, the volume of the first chamber R1 is reduced and the volume of the second chamber R2 is expanded by the piston 103 that is moved down in conjunction with the spindle 3. Accordingly, the pressure in the first chamber R1 is increased while the pressure in the second chamber R2 is decreased. Thus, the piston 103 is braked, whereby a damper effect can be obtained (i.e., a force against a biasing force or the like of the biasing unit applied on the spindle 3 is applied on the spindle 3). Consequently, the descent of the spindle 3 can be braked. Incidentally, when the dial gauge is laid sideways, the ball 108 is slipped from the valve seat 107 to open the through hole 106. Accordingly, the braking force can be weakened corresponding to decrease in a component of gravity axially applied on the spindle 3 and the piston 103
However, in the braking device 100 disclosed in Document 1, it is required that the first chamber R1 and the second chamber R2 that are hermetically sealed are provided above the body case 2 into which the upper end of the spindle 3 is inserted. Thus, a lifting mechanism for the spindle 3, i.e., a lifting mechanism for lifting the spindle 3 for measurement, cannot be provided.